Markdown Visualizer is where I write READMEs and changelogs before they go anywhere public. Editor on the left, preview on the right, and the preview keeps up with every keystroke.

It started on 4 March 2026, the same day I tore down and rebuilt JSON Visualiser. The second commit says it plainly: “init markdown visualizer bearing torch from json vis”. The workspace, the persistence, and the keyboard habits all came across. Only the format changed.

The live app. Type in the editor and the preview renders as you go.

The preview

The preview renders through Streamdown, with its code and Mermaid plugins added on day two. Fenced code is highlighted, and a mermaid block becomes a real diagram right next to the prose that explains it:

```mermaid
flowchart LR
  Write --> Preview
  Preview --> Ship
```

Paste that into the editor above and the flowchart draws itself.

Streamdown was not a random pick. In September 2025, months before this app existed, I contributed table copy and CSV download and code block downloads to it. Building on a renderer I had already worked inside made the preview the easy part.

Panes

Ten days in, I rebuilt the layout as two resizable panes, each one expandable to full width. A week after that came shortcuts, so I could stop reaching for the mouse:

Shortcut	Action
`Cmd` `E`	Expand the editor, press again to split
`Cmd` `Shift` `P`	Expand the preview, press again to split
`Cmd` `D`	Toggle light and dark

Then I removed the header bar from each pane. Without it, the panes are just the text and the rendered page.

On a phone the split becomes two tabs, Editor and Preview, so neither side gets squeezed.

Drafts survive

Each tab saves its document to IndexedDB as you type. Reload, close the laptop, come back, and the draft is still there. It installs as a progressive web app and keeps working offline.

There is no account, and no backend of its own that stores your writing.

Open Markdown Visualizer and draft your next README.